Many mechanical devices and processes generate heat. Fluids are typically used in the apparatus or in the process to absorb the heat. Heat exchangers are then used to cool the process fluid so that the apparatus and the process operate in an appropriate range of temperatures. For example, applications requiring transfer of heat from a process fluid to a refrigerant fluid include computers, laser equipment, medical equipment, automobiles, air conditioning, waste and process heat recovery, aircraft, and the like. Compact heat exchange surfaces are used in these applications to extract heat from a process fluid. The fluids involved may be gas or liquid.
Compact heat transfer apparatus have been constructed for use with such heat-generating apparatus and processes. These heat transfer apparatus are typically known as heat exchangers. Heat exchangers include a plurality of heat transfer surfaces for exposing the heated process fluid to the refrigerant fluid. Compact heat exchangers are generally characterized as having extended surfaces for transfer of heat. The most common configurations are known in the industry as either a plate-fin or a tube-fin type of surface. The fins provide significantly large surface area-to-volume ratios to facilitate heat transfer. Although both types of heat exchangers function to extract heat from the process fluid, the types have significant differences. Of the two types, plate-fin devices typically exhibit a significantly higher ratio of heat transfer surface to volume of heat exchanger. The process fluid flowing through the heat exchanger typically is in laminar flow against the heat transfer surfaces. Laminar flow enhances heat transfer by conduction across the fluid film on the plate.
Tube-fin type heat exchangers in contrast typically exhibit turbulent flow in the process fluid. The turbulent flow is imposed by the shape and the relatively off-set relationship of the tubes that carry the refrigerant fluid. Turbulent flow impedes heat transfer by conduction across the inherent film boundary.
Typically, either type is found in a single stage heat exchanger which is designed for the specific requirements of the apparatus and process to be cooled. The heat exchangers provide the plurality of tubes or plates through which the refrigerant fluid flows. The refrigerant may be returned across the flow path of the process gas typically once or perhaps twice. The number of plates or tubes depend on the cooling capacity required for the apparatus or process.
Heat exchangers typically comprise a housing having a process fluid inlet and a process fluid outlet. The process fluid typically flows in a cross-direction to the refrigerant flow through the plates or tubes. A heat exchanger having two or more sections, or stages, of stacked plate or tube elements has been described previously. For example, U.S. Pat. No. 3,746,084 describes a housing that encloses several sections of stacked refrigerant pipes. Each section includes a separate input and output header for the refrigerant supplied to that section. This heat exchanger, however, does not provide a rigorous flow path for the process fluid. The header is exposed to the process fluid.
While functioning to extract heat from the process fluid, the heat exchanger has several drawbacks. Nonuniformity of heat extraction leads to purity problems of the resulting cooled process fluid. Heat exchangers particularly are used in condensers which extract waste liquid and contaminants and thereby purify the process fluid. The nonuniformity of the thermal transfer and the unrigorous flow path for the process fluid may result in untreated process fluid moving through the heat exchanger. Untreated process fluid retains the contaminants and thus the purity of the resulting process fluid is not as great. For example, methane gas typically emits from a land fill. The gas is generated by decaying debris placed in the landfill. The methane gas further contains contaminants that are emitted by the decaying materials. These contaminants include chlorinated hydrocarbons, aromatics, and organic silicon compounds, among others. The heat exchanger cools the methane gas to condense water from the gas. The contaminants condense into the water. The water drains from the heat exchanger to a collector. A separator may be used to divide the contaminants for separate collection. The open design and nonuniform thermal transfer may lead to short circuiting and inefficient or ineffective cooling of the process fluid.
As discussed above, the size of the heat exchanger is dependent upon the capacity required for treating the process fluid. To facilitate economies of scale, standard sized heat exchangers are typically available from manufacturers. The heat exchanger is selected based on using the smallest capacity to meet the expected BTU per hour extraction requirements of the apparatus or process. This results in the heat exchanger typically having over-capacity for the expected work. This provides a margin of error for the heat exchanger. However, over-capacity contributes to inefficiencies in operation. For example, the pressure drop of the process fluid passing through the heat exchanger may be significantly great that additional pumping mechanisms are necessary to maintain the flow of the process gas.
Thus, there is a need in the art to provide heat exchange modules that are grouped to define a rigorous flow path for treating a process fluid with refrigerant fluid supplied to the heat exchange modules in parallel, for increasing the uniformity of heat exchange across the flow path.